Portugal will face Iran on Monday in Saransk in their final Group B match. Both teams have a chance to reach the knockout stage, as well as Spain, who will play against Morocco on the same day.
"Tomorrow will be a wonderful, spectacular game. Both teams want to win, we must be ready for a tough fight, but I think we will make it to the round of 16," Santos said at the pre-match press conference.
He added that Iran were a strong and well-organized team, stressing that they did not focus on defense alone and "know how to counter-attack."
If Portugal make it to the knockout stage, they will play against Uruguay or Russia from Group A.
The 2018 FIFA World Cup kicked off on June 14 and will continue until July 15, with the matches being played in 11 cities across Russia.